Improvement in decontamination of aqueous acidic solutions containing plutonium and fission product values by providing cerous and/or mercuric ions therein prior to a bismuth phosphate carrier precipitation



United States Patent Burt F. Paris and Harry K. Strassel, Richland,Wash., as-

slgnors to the United States of America as represented by the UnitedStates Atomic Energy Commission I No Drawing. Filed Mar. 31, 1945, Ser.No. (586,038

5 Claims. (Cl. 23-145) This invention relates to a procedure for theprocessing of materials containing the element of atomic number 94,known as plutonium, for separating the plutonium from extraneous mattersuch as substances of the kind present in neutron irradiated uraniumexemplified by uranium and especially fission products, and the likeradioactive contaminants. More particularly, this invention concerns aseparatory and concentration procedure involving the use of supplementaladditions, such as mercury and cerium agents, whereby improveddecontamination and Pu recovery may be obtained.

As described herein, the isotope of element 94 having a mass of 239 isreferred to as 94 and is also called plutonium, symbol Pu. In addition,the isotope of element 93 having a mass of 239 is referred to as 93Reference herein to any of the elements is to be understood as denotingthe element generically, whether in its free state or in the form of acompound, unless indicated otherwise by the context.

Elements 93 and 94 may be obtainedfrom uranium by various processeswhich do not form a part of the present invention including irradiationof uranium with neutrons from any suitable neutron source, butpreferably the neutrons used are obtained from a chain reaction ofneutrons with uranium.

Naturally occurring uranium contains a major. portion of U a minorportion of U and small amounts .of other substances such as UX and UXWhen a mass of such uranium is subjected to neutron irradiation, par- 1ticularly with neutrons of resonance or thermal energies, U by captureof a neutron becomes U which has a half life of about 23 minutes and bybeta decay becomes 93 The 93 has a half life of about 2.3 days and bybeta decay becomes 94 Thus, neutron irradiated uranium contains both 93and 94 but by storing such irradiated uranium for suitable period oftime, the 93 is converted almost entirely to 94 In addition to theabove-mentioned reaction, the reaction of neutrons with fissionablenuclei such as the nu- .cleus of U results in the production of a largenumber of radioactive fission products. As it is undesirable to producea large concentration of these fission products which must, in view oftheir high radioactivity, be separated from the 94 and further as theweight of radioactive fission products present in neutron irradiateduranium is proportional to the amounts of 93 and 94 formed therein, itis preferable to discontinue the irradiation of the uranium by neutronswhen the combined amount of 93 and 94 is equal to approximately 0.02percent by weight of the uranium mass. At this concentration of thesesubstances, the concentration of fission elements which must he removedis approximately the same percentage.

A number of processes have already been proposed for ice accomplishingthe separation and concentration of Pu. Certain of these processes aregenerically known as the bismuth phosphate type process and the wetfluoride" type. of process. These processes are the invention ofothersand the details of the processes are described in copendingapplications, as for example application Ser. No. 519,714, filed January26, '1944, now US. Patent 2,785,951, issued March 19, 1957, to bereferred to here inafter, which gives details relative to suchprocesses.

It is sufiicient to indicate at this point that by means of suchprocesses, of which there are a number, applied in one or several cyclesdependent upon the concentration of contaminants and similarconsiderations that thecontamination may be substantially reduced andthe plutonium recovered. That is, as described above the desiredplutonium to be recovered is in the presence of substantial quantitiesof fission products evidenced by beta and gamma activities, and theplutonium must be isolated from at least a portion of the activity. Inother words, the activity must be eliminated either completely or atleast to a. certain tolerance level so that the Pu may be handled orfurther used without danger from radioactivity due to contaminants. Thebismuth phosphate method of by-product and product precipitation isgenerally illustrative of processes which may be applied to separate andrecover plutonium.

While existing processes, includingthe bismuth phos phate type ofprocess, are quite efficient and successful, it may be necessary torepeat a process a numberof times in order to obtain a Pu in which theactivity .has been reduced to the tolerable amount. This may beaccomplished by repeated application of product and by-product cycles aswill be referred to in further detail hereinafter. In some instances,the feed materials to be processed may be higher in both plutoniumcontent and much higher in gamma activity than materials usuallyprocessed. Consequently, there is a need for supplemental procedures inorder to more quickly and thoroughly redu'ce oreliminate activities.

We have found that the aforementioned processes which are referred toherein as existing, standard or conventional processes, exemplified bythe bismuth phosphate process, may have included therein certainsupplemental agents and operations for materially improvingdecontamination. This improvement may be indicated by obtaining afavorable decontamination factor in fewer cycles, or by obtaining ahigher decontamination factor, namely, more complete decontaminationthan formerly possible by the particular process in the same number ofsteps.

The meaning of the terms bismuth phosphate type of process, product andby-product precipitate, decontamination, and similar terms are apparentto some extent from the preceding description and will be furtherapparent as the description proceeds.

The invention has for one object, to provide improvements in a methodfor the separation and recovery of plutonium.

Another object is to provide a method of separating plutonium byprocedure wherein certain agents and steps supplemental to those,hereintofore used are employed.

Another object is to provide improved procedure for eliminating,irradicating or reducing gamma contamination.

Still another object is to provide improved procedures particularlyuseful in the bismuth phosphate type of proc- 6S5.

Another object is to provide improved procedures in forming bothby-product and product carrier precipitates.

Still another object is to provide a procedure for improvingdecontamination.

Still a further object is to provide steps and reagents which lendthemselves to combination with steps already known or practiced.

Another object is to provide a type of process which may employmaterials used in existing processes, but with more effectivedecontamination, and which may be carried out in existing equipmentwithout change, or with a minimum of equipment change.

Still another object is to improve existing procedures by reducing thenumber of cycles required for decontamination or obtaining higherdecontamination by the same number of cycles.

Another object is to provide a process susceptible of treating sourcesof Pu containing concentrations thereof and of activities higher thanheretofore usually encountered.

Other objects will appear hereinafter.

We have found that Pu in admixture with various extraneous material maybe separated and concentrated by the use of the series of stepsinvolving the formation of certain carrier precipitate type oftreatments. These treatments may be similar to existing practice in manyfeatures as respects some of the reagents, temperatures and similaraspects. However, we have found that the function of such carrierprecipitates of carrying contamination away from Pu and carrying Pu awayfrom contamination may be rendered considerably better by the inclusionof certain reagents and steps as set forth in detail herein. By theformation of the carrier precipitate under the conditions of the presentinvention, not only are the advantages, such as good separation of Pupreviously obtained in the processes, still obtainable, but additionaladvantages such as improved gamma decontamination are obtained.

As referred to above, an illustration of the types of carrierprecipitates involved are described in application Ser. No. 519,714,filed January 26, 1944, Thompson and Seaborg, now US. Patent 2,785,951,issued March 19, 1957, and reference is made to that application forfurther disclosure, details thereof being omitted from the presentdisclosure except where necessary to an understanding of the presentinvention. As set forth in said application, it has been discovered thatplutonium has more than one oxidation state, including a lower oxidationstate or states referred to herein as Pu in which the element ischaracterized by forming insoluble phosphates and other insolublecompounds and a higher oxidation state or states referred to as Pu inwhich the element forms soluble phosphates and other soluble compounds.As further set forth in said application Ser. No. 519,714, the solubleplutonium phosphate in which plutonium is present in its higheroxidation state is referred to as plutonyl phosphate, (PuO (PO Putherein having a valence of six, and the valence of plutonium in itsphosphate insoluble state is not greater than four.

That is, by having the metal as Pu" a product precipitation orextraction may be accomplished in which a carrier precipitate bringsdown the Pu" leaving behind in solution a substantial portion of fissionproducts and other sources of contamination. However, some of theactivity may be carried along with the precipitate or otherwise may beoccluded with the Pu" Certain contaminants such as columbium, zirconium,and lanthanum activities may be quite difiicult to separate from -Pu.

The carrier precipitate containing Pu" after suitable dissolution, isoxidized so that Pu is obtained which remains in solution and by meansof a by-product carrier precipitate, such as a bismuth phosphateby-product precipitate, under the oxidizing conditions carriesdownfission products leaving the Pu in solution. However, in this stepcertain by-pro'duct precipitate fines may not separate out or otherwisesome contamination remains so that the solution remaining which containsthe Pu may still be contaminated with some fission products although inmuch smaller amounts. It should be kept in mind that since the Pu is inan environment of radioactivity which may give many million of counts aminute per milligram of fission product present, that the clean-cutseparation of the Pu presents considerable difficulty.

Illustrative details respecting bismuth phosphate byproduct and productprecipitation will be apparent from the following: Neutron irradiateduranium is dissolved in a suitable quantity of 60-70% nitric acid. Thisgives a solution containing Pu such as may be referred to as a UNH(uranyl nitrate hexahydrate) solution. The solution is subjected totreatment with an agent such as sodium nitrite, formic acid, H 0 oxalicacid or the like in excess for a period of about one hour at atemperature from 50 C. to 75 C. whereby any of the Pu which may havebeen oxidized to the Pu state in the solution step is reduced to the Pustate. The concentration of the solution in the UNH is adjusted to 20%and H fluo silicic acid or the like is added to make the solution 1 Ntherein. To-the solution is now added a source of bismuth ion to providea concentration of bismuth ion equivalent to 10 grams of Bi+ ion in fourliters of 20% UNH; phosphoric acid is also added to make the solutionbetween .3.8 M therein, and a precipitate comprising BiPO which carriesthe Pu comes down and is separated from the solution by filtration orcentrifugation. This illustrates a product precipitation. The BiPO,precipitate carrying the Pu" is dissolved in 10 N HNO The acidity of thesolution is reduced to 6 N HNO by dilution and the solution made .1 M inK Cr O Various other oxidizing agents such as sodium bismuthate may alsobe used. On heating the solution at 95 C. for 2.25 hours, the plutoniumis oxidized to the Pu state. The solution is then diluted to l N acidityby addition of water and H PO added to provide a suitable concentrationfor causing the formation of a BiPO precipitate. The solution is heatedto about C. whereupon BiPO precipitates carrying fission products butnot Pu The precipitate may be removed by filtration or centrifugationand discarded. This illustrates a by-product precipitation. Ifrepetition of the cycle is contemplated for further decontamination, thePu in the filtrate is reduced by passing in a rapid stream of S0 gas forfive minutes and allowing the solution to stand for approximately onehour, or by oxalic acid, Fe+ or similar reduction, and the cycle issuitably repeated. 7

The exact manner of obtaining the bismuth phosphate product andby-product precipitates is not a limitation on the present invention.The product precipitation under (r) conditions, as indicated, may beaccomplished by using any of a number of reducing agents of which sulfurdioxide, hydrogen peroxide, oxalic acid, sodium nitrite, ferrous iron,and the like are mentioned for illustration. Likewise the by-productprecipitation under (0) conditions, as indicated, may be accomplished bya number of oxidizing agents for obtaining the (0) state. Dichromates,sodium bismuthate, permanganate and the like are mentioned forillustration. Various amounts and sources of bismuth or phosphate ionsmay be used. Details respecting these aspects are described in othercopending applications and form no integral part of the presentinvention. Consequently, in the description which follows reference willmerely be made to a bismuth phosphate product or by-product carrier, orsimilar terms used, it being apparent that such carrier may be'formed inany suitable manner.

The solutions containing Pu which may be treated by our invention may bethe same type of solutions as heretofore treated, or may be solutions inwhich the concentrations of components are much higher. As apparent fromthe foregoing, one common type of solution containing Pu subject toseparation and recovery procedures are the solutions processible by abismuth phosphate type. of

treatment. As indicated, such solutions comprise a nitric acidcontaining liquid having a content of Pu therein. The nitric acidsolution may also contain other'materials such as a content of variousother acids. The solution may also contain certain extraneous mattersuch as radioactive materials which the procedure of the presentinvention eliminates or reduces.

We have found that by means of certain supplemental additions, such as acerium addition applied to the solution containing the Pu namely in thereduced condition, the carrier (product precipitate) better carries thePu away from extraneous matter. By means of certain supplementaladditions, such as a mercury addition, applied to the solution havingthe desired element Pu in a state of higher oxidation (by-productprecipitate) extraneous matter is carried away by the precipitateleaving the Pu in the supernatant liquid from this precipitation therebydecontaminated to an improved extent.

Broadly, therefore, our invention may be applied to various solutionsand particularly solutions containing high activities. The solutions tobe processed by our invention have added thereto such reagents'as may berequired, dependent upon the particular composition and condition of thesolution, to obtain the customary carrier precipitate formation at thatparticular phase of the process. Before, simultaneously therewith, orthereafter as will be further apparent from the examples, there isapplied thereto the procedure of the present invention for causingimproved decontamination, particularly the elimination of gammaactivity.

In general, the operation of our process as respects its use inconjunction with bismuth phosphate precipitates is as follows: We havefound that the addition of a source of mercury ions such as mecuricnitrate, preferably added before a bismuth phosphate by-productprecipitation step, improves decontamination. That is, the addition ofHg++ increases the elimination of gamma activity in both the by-productand the product precipitation step. There would also be added before thenext product precipitation step a source of cerium ions. By using acombination of agents such as mercury in the byproduct precipitation andcerium in the product precipitation step, for example, a four-foldincrease in gamma decontamination may be efiected. Also the plutoniumseparation in the product precipitation step is better. Approximately,1-5 grams of Hg++ per liter of solution treated and 1-5 grams of Ce+++are illustrative of quantities of these supplemental additions which maybe used under most conditions.

A still further understanding of our invention will be had by referenceto the following detailed examples. In these examples the solutionscontaining Pu which was treated were, in general, the usual typeencountered industrially, excepting that the gamma activity was in manyinstances higher than usual. That is, the solutions were obtained fromstandard processing and were approximately 1 normal in nitric acid. Thesolution also may contain residual oxidizing of reducing agents, variousinorganic acid and the like ingredients in addition to extraneousmaterial, such as high activities.

EXAMPLE I In this example a nitric acid solution of the abovedescribedtype was treated and agents of the present invention were added in twoplaces. One hundred cubic centimeters of starting solution (extractionproduct dissolved in N HNO 2.5 g./l. Bi as BiPO was oxidized and 5 g./l.Hg, as mercuric nitrate, was added before a BiPO by-productprecipitation was made. The byproduct filtrate was reduced and 2 g./l.tartaric acid added, and the BiPO final product precipitation made.

The results of the example indicated improvement in decontamination.Almost twice as much gamma activity was taken out by the by-productprecipitation, only 12.0% of the remaining gammas were carried by thefinal product precipitation, and the gammas remaining at the V 6 r 7 endof the first cycle were only /5 the number remaining in the control. Thepercent yield as respect Pu was satisfactory, being 93.5%.

EXAMPLE II In order further to study the effect of the additions asingle addition was made. One hundred cc. of starting solution wasoxidized, a by-product precipitate made, and the filtrate reduced in thesame manner as the control. To the reduced solution was added 2 g./l.tartaric acid and a final bismuth phosphate product precipitate made.

Tartaric acid added at this point did not interfere with the MP0;carrying. The percent gamma carried by the final product precipitationwas 26.4%, namely more than twice that in the preceding example, and thepercent Pu yield, 95.8%. By a comparison of this example with thepreceding example, it is apparent that the presence of Hg+ in thepreceding example substantially increased the decontamination in theby-product step and also aided in the final productionprecipitationstep.

EXAMPLE III Inasmuch as Hg and Ce are regarded as the preferredcombination, a series of nine runs were made to determine the optimumweight of Hg and Ce to be used as addition agents. The Hg was used inthe by-product precipitation step and added as Hg(NO just prior toprecipitation, and the Ce was added just prior to the final productprecipitation.

The runs were made on a 230 g. UNH/l. solution containing rather highactivity. With the exception of the addition agents, existing procedureswere followed for extraction plus one BiPO decontamination cycle.

The Hg and Ce were added in the following weight combinations: (1) 7 /2g. Hg/l.-l g. Ce/L, (2) 7 /2 g. Hg-5 g. Ce, (3) 7 /2 g. Hg10 g. Ce, (4)2 /2 g. Hg 1 g. Ce, (5) 2 /2 g. I-Ig5 g. Ce, (6) 2 /2 g. Hg10 g. Ce, (7)1 g. Hg0.0 g. Ce, (8) 1 g. Hg-l g. Ce, (9) l g. Hg5 g. Ce.

Analysis by counting and a summary of the results were made, and thisindicated that increases in decontamination were obtained in all cases.The agents did not atfect Pu carrying. It appeared that 1 g./l. each ofHg and Ce is a satisfactory combination, although both larger andsmaller amounts were operative and gave improved decontamination. Ingeneral, it is preferred to use the minimum amounts consistent withobtaining the desired decontamination. This conserves materials by notusing excesses. Also any possibility of reagent appearing in therecovered Pu is minimized.

EXAMPLE IV While, for various reasons the combination designated asHg-Ce, is preferred, there are other combinations in accordance with thepresent invention which may be used. Several studies .were made todevelop combinations that would function in a manner somewhat comparableto the Hg-Ce combination in aiding decontamination. Four runs were madein which 1 g. Hg+ per liter was added just prior to the by-productprecipitation, and 3.2 g. diglycolic acid, 3.2 g. tartaric acid, 3.0 g.Get and 3.0 g. Mn+ per liter, respectively, added justprior to the finalproduct precipitation.

The source of Pu which comprised the 230 g. UNH/l. type solutioncontaining high fission activity was used as the starting solution. Withthe exception of the addition agents, standard procedure was followedfor extraction plus one BiPO decontamination cycle.

, All the decontamination factors ran low presumably due to the factthat a sedimation had occurred in the starting solution and wasincorporated into the runs. However, the data indicated that theHg-tartaric acid combination and the Hg-Mn combinations comparedfavorably with" the Hg-Ce combination.

Some of the analytical data and results are tabulated in the followingtable: a

Data has been presented in the preceding description establishing thatdecontamination improvement may be obtained by the use of variousreagents. In summarizing, it may be indicated that preferred operationsare as follows: The addition of Hg++ prior to BiPO precipitationincreases carrying of fission (gamma) activity which is beneficial inthe by-product step, but may have an adverse elfect in the productprecipitation step. However, the use of Ge (probably as a holdbackcarrier) gives some decontamination improvement in the productprecipitation step. Hence, by the combination of agents significantdecontamination improvement may be obtained such as by the use of Hg inthe by-product step and with Ce+ added in the product precipitationstep. This procedure may also be used in conjunction with existingprocesses in which scavengers, such as Ce-Zr phosphates are used. Datagiven in the following table shows that the addition agents of thepresent invention had some additive effect when used in conjunction witha bismuth phosphate process in which scavengers were also used.

Table EFFECT OF ADDITION OF Hg AND Ce ONv DECONTAMINATION (GAMMAACTIVITY) Grams Hg++/l. Grams Ce+ By- Product Overall added before addedbefore product pptu. Factor by-product prod. pptn. decontam decontarufor pptn. factor factor Cycle 2. 1 10. 3 l0. 3 106 2. 5 5 l0. 3 9. 7 1002. 5 10.3 6. 1 62. 8 7. 5 1 l2. 6 9. 8 123 7. 5 5 l2. 6 10. 7 135 7. 510 12. 6 9. 0 113 1 0 8. 5 12. 3 105 1 1 8. 5 11. 4 97 1 5 8. 5 18. 9161 5 0 l2. 9 5. 3 64. 1 0 5 4. 9 10. 6 52 0 0 3. 3 8. 2 27 While theprocess has been described in particular with respect to the phosphatetype of by-product and product precipitates, illustrated by alternatebismuth phosphate precipitates since this constitutes the preferredembodiment as well as a type of precipitate frequently encountered inplant processes, our invention is not limited thereto. Similar proceduremay be applied in the formation of other precipitates which function ina similar manner under alternate by-product and product precipitationsteps. That is, in other instances where precipitates do not adequatelydecontaminate or otherwise present difiiculties, they may be improved bythe application of the principles of our invention.

It may be seen from the foregoing that we have provided certainsupplemental additions to by-product and product precipitation stepswhich produce substantial improvement in the overall decontamination andalso permit a better separation of plutonium. The preferred procedurecomprising adding a source of mercury in the byproduct step and a sourceof cerium in the product precipitation. However, as also indicated,other combinations may be employed, such as adding mercury in theby-product step and tartaric acid or comparable organic reagents in theproduct precipitation.

While mercury has been described as added as the nitrate since nitricacid solutions are being treated and the use of the nitrate eliminatesadding other ions to the solution, our invention does not preclude theuse of other compatible sources of material such as mercuric phosphate,or the like. Similar comments apply to the particular source of ceriumreferred to herein as cerium nitrate. Other cerium salts such as ceriumtartarate or citrate and the like may be employed in place of, or inaddition to cerium nitrate.

The term decontamination factor used herein is a comparative value orratio derived by dividing the count value, or average for a series ofruns, before any particular step by that count value obtaining after thestep. An overall factor is the value obtained by multiplying togetherseveral individual factors. As apparent from the preceding examples, acycle comprises a BiPO by-product precipitation coupled with a BiPOproduct precipitation.

As described above the by-product, product precipitation and othersimilar steps used such as a BiPO, product precipitation are, per se,the invention of others, the present invention constituting animprovement of existing practices. Hence, the details respecting suchsteps are not a limitation on the present invention. In the examplesdescribed herein the particular details respecting forming the carrierprecipitates were the same for both the processing in accordance withthe present invention and for the control.

For example, to obtain a MP0,, carrier precipitate while the normalityof the environment where the precipitate is formed has been indicated atabout 1 N, it may vary substantially as from below 1 N and up to 2.5 N.For example, the choice of a particular concentration of such acid incarrying out the present invention may be influenced by corrosion effecton the particular piece of metal equipment in which the step is carriedout. The concentration of bismuth may vary as from .5 gram per liter to3 or 4 grams per liter. Losses may be slightly higher with the loweramount. Likewise, the concentration of phosphoric acid may vary as from.3 M to l M, around .6 M being a generally satisfactory value. Thedigestion period and temperature for the MP0,, precipitate, namely theperiod allowed for the precipitate to gather or otherwise form prior tocentrifuging or filtering, may vary from 50 C.l00 C. for one-half toseveral hours. Likewise, the technique such as rates and manner ofadditions are not fixed procedures. In general, as already discussed,the use of the smallest amounts consistent with good operation ispreferred in order to avoid waste through excesses or for similarreasons. However, the use of larger amounts is not precluded.

The process may be applied to solutions containing Pu from traceramounts to several hundred grams thereof. Also large volumes of liquidsmay be treated. The activities of the solutions may be many times higherthan the activities usually encountered in the prior art. However, ourinvention is not limited in these respects as the concentrationssuggested are merely guides.

It is to be understood that all matter contained in the abovedescription and examples shall be interpreted as illustrative and notlimitative of the scope of this invention, and it is intended to claimthe present invention as broadly as possible in view of the prior art.

We claim:

1. In a process for the recovery of plutonium from aqueous acidicsolutions containing plutonium ions and ions of contaminating fissionproducts, which includes the steps of precipitating bismuth phosphate ina solution containing plutonium ions in an oxidation state no greaterthan four and separating the resulting precipitate and its associatedplutonium from the supernatant solution, and the steps of precipitatingbismuth phosphate in a solution containing hexavalent plutonium ionstogether with ions of contaminating fission products and separating theprecipitate and its associated fission products from the supernatantsolution, the improvements which comprise providing a source of cerousions in the said solution of plutonium ions in an oxidation state nogreater than four prior to precipitating thebismuth phosphate, andprovid ing a source of mercuric ions in the said solution of hexavalentplutonium ions prior to precipitating the bismuth phosphate.

2. In a process for the recovery of plutonium from an aqueous acidicsolution containing plutonium ions and ions of contaminating fissionproducts, the steps which comprise maintaining the plutonium in saidsolution in an oxidation state no greater than four, providing a sourceof cerous ions in said solution, forming a bismuth phosphate precipitatein said solution, separating the bismuth phosphate precipitate and itsassociated plutonium from the supernatant solution, dissolving theseparated precipitate and its associated plutonium in an aqueous acidicsolution, maintaining the dissolved plutonium in said solution in thehexavalent state, providing a source of mercuric ions in said solution,forming a bismuth phosphate precipitate in the resulting solution, andseparating the precipitate and its associated fission products from theplutoniumcontaining supernatant solution.

3. In a process for the recovery of plutonium from an aqueous acidicsolution containing plutonium ions and ions of contaminating fissionproducts, the steps which comprise maintaining the plutonium in saidsolution in the hexavalent state, providing a source of mercuric ions insaid solution, forming a precipitate oi bismuth phosphate in theresulting solution, separating the bismuth phosphate precipitate and itsassociated fission products from the supernatant solution, reducing theplutonium in the supernatant solution to an oxidation state no greaterthan four, providing a source of cerous ions in said solution, forming aprecipitate of bismuth phosphate in the resulting solution, andseparating the bismuth phosphate pre- 10 cipitate and its associatedplutonium from the supernatant liquid.

4. In a process for decontaminating an aqueous acidic solutioncontaining plutonium ions and ions of contaminating fission products,the steps which comprise maintaining the plutonium in said solution inthe hexavalent state, providing a soin'ce of mercuric ions in saidsolution, forming a bismuth phosphate precipitate in the resultingsolution, and separating the bismuth phosphate precipitate and'itsassociated fission products from the plutonium-containing supernatantsolution.

5. In a process for decontaminating an aqueous nitric acid solutioncontaining plutonium ions and ions of contaminating fission products,the steps which comprise maintaining the plutonium in said solution inthe hexavalent state, providing in said solution 1-5 grams of mercuryper liter in the form of mercuric ions, forming a bismuth phosphateprecipitate in the resulting solution, and separating the bismuthphosphate precipitate and its associated fission products from theplutonium-containing supernatant solution.

References Cited in the file of this patent UNITED STATES PATENTS2,776,185 Werner et a1. Jan. 1, 1957 2,785,951 Thompson et a1. Mar. 19,1957 2,799,553 Thompson et a1. July 16, 1957 OTHER REFERENCES Seaborg:The Chemical and Radioactive Properties of the Heavy Elements, Chemicaland Engineering News, volume 23, pages 2190-2193 (December 10, 1945).

Harvey: The Actinide Elements and the Chemistry of Plutonium,Nucleonics, vol. 2, No. 4, pages 30-40, particularly page 34 (1948).

1. IN A PROCESS FOR THE RECOVERY OF PLUTONIUM FROM AQUEOUS ACIDICSOLUTIONS CONTAINING PLUTONIUM IONS AND IONS OF CONTAMINATING FISSIONPRODUCTS, WHICH INCLUDES THE STEPS OF PRECIPITATING BISMUTH PHOSPHATE INA SOLUTION, CONTAINING PLUTONIUM IONS IN AN OXIDATION STATE NO GREATERTHAN FOUR AND SEPARATING THE RESULTING PRECIPITATE AND ITS ASSOCIATEDPLUTONIUM FROM THE SUPERNATANT SOLUTION, AND THE STEPS OF PRECIPITATINGBISMUTH PHOSPHATE IN A SOLUTION CONTAINING HEXAVALENT PLUTONIUM IONSTOGETHER WITH IONS OF CONTAMINATING FISSION PRODUCTS AND SEPARATING THEPRECIPITATE AND ITS ASSOCIATED FISSION PRODUCTS FROM THE SUPERNATANTSOLUTION, THE IMPROVEMENTS WHICH COMPRISE PROVIDING A SOURCE OF CEROUSIONS IN THE SAID SOLUTION OF PLUTONIUM IONS IN AN OXIDATION STATE NOGREATER THAN FOUR PRIOR TO PRECIPITATING THE BISMUTH PHOSPHATE, ANDPROVIDING A SOURCE OF MERCURIC IONS IN THE SAID SOLUTION OF HEXAVALENTPLUTONIUM IONS PRIOR TO PRECIPITATING THE BISMUTH PHOSPHATE.